The present invention is directed to systems and methods for embedding information into an image and more particularly to systems and methods for embedding invisible information into an image and reliably decoding the information at a later time.
The technique of embedding encoded information into paper documents using data glyph technology has been widely practiced for the last 20 years. It is particularly advantageous for use in document applications that require a high density rate of embedded data and require the embedded data to be robust for decoding purposes. However, data glyph encoding produces perceptible image changes which may detract from the overall image quality of a document.
Data glyph technology encodes digital information in the form of binary 1""s and 0""s that are then rendered in the form of distinguishable shaped marks such as very small linear marks. Generally, each small mark represents a digit of binary data; whether the particular digit is a digital 1 or 0 depends on the linear orientation of the particular mark. For example, in one embodiment, marks that are oriented from top left to bottom right may represent a xe2x80x9c0,xe2x80x9d while marks oriented from bottom left to top right may represent a xe2x80x9c1xe2x80x9d. The individual marks are of such a small size relative to the maximum resolution of a black and white printing device so as to produce an overall visual effect to a casual observer of a uniformly gray halftone area when a large number of such marks are printed together in a black and white image on paper. When incorporated in an image border or graphic, this uniformly gray halftone area does not explicitly suggest that embedded data is present in the document. However, a viewer of the image could easily detect that the small dots forming the gray halftone area are a series of small marks that together bear binary information.
History has shown that the closely spaced glyphs forming the gray halftone area are often processed incorrectly by descreening algorithms utilized by fax machines, copiers and scanners. More specifically, during a typical copy or scan operation, some descreening algorithms attempt to provide some enhancement of an image by smoothing and averaging data within image regions. Similarly, during a typical fax operation, the resolution of the fax machine will smooth and average data. Since prior art glyphs are so close together, the descreening process results in the inadvertent erasure of some glyphs.
Examples of U.S. Patents on data glyph technology are U.S. Pat. Nos. 5,221,833, 5,245,165, and 5,315,098. U.S. Pat. No. 5,221,833, entitled xe2x80x9cMethods and Means for Reducing Error Rates in Reading Self-Clocking Glyph Codesxe2x80x9d, discloses a method for encoding n-bit long multi-bit digital values in a pre-ordered cyclical sequence based on their analytically or empirically determined probabilities of being confused with each other, such that each glyph is adjacent in that sequence to the two glyphs with which it is more likely to be confused during decoding. U.S. Pat. No. 5,245,165, entitled xe2x80x9cSelf-Clocking Glyph Code for Encoding Dual Bit Digital Values Robustlyxe2x80x9d, discloses a method for encoding dual bit digital values in the cardinal rotations (0xc2x0, 90xc2x0, 180xc2x0 and 270xc2x0) of a logically ordered sequence of wedge-shaped glyphs (essentially right triangles) that are written, printed or otherwise recorded on a hardcopy recording medium with a predetermined spatial formatting rule. The widths of the glyphs vary unidirectionally as a function of their height, so they can be decoded reliably, even when they are degraded by scan errors, dropped scan lines and/or random noise patterns.
A system and method for encoding digital data in halftone images is disclosed in U.S. Pat. No. 5,315,098 to Tow, entitled xe2x80x9cMethods and Means for Embedding Machine Readable Digital Data in Halftone Images,xe2x80x9d the disclosure of which is incorporated herein by reference in its entirety. In Tow, digital data are encoded in the angular orientation of circularly asymmetric halftone dot patterns that are written into the halftone cells of digital halftone images. The sizes of the halftone dot patterns are modulated in accordance with grayscale data sample values that are provided to define the image. The patterns are modulated so that the average reflectance or transmittance of each of the halftone cells is modulated to provide a more or less standard halftone rendering of the image. By modulating the angular orientation of the halftone dot patterns, digital data is encoded within the halftone image. The digital data can then be scanned into a computer, decoded and later processed. However, the close spatial relationship between adjacent halftone dot patterns may complicate the process of discriminating between logical states. Tow therefore set aside crosshatched pixels to function as dedicated background pixels to simplify the task of discriminating between their different angular orientations. Unfortunately that also resulted in visible graininess and degradation of image quality. Accordingly, there is a need for a less obtrusive code that is reliably decoded without affecting image quality.
In the prior art, the modulation of the angular orientation of the halftone dot patterns may disadvantageously affect the quality of the image because even with a low pixel density (e.g., 2%), the halftone images still detract from the appearance of the overall image because they have visible graininess to the naked eye. In addition, usual processing by fax, scan, or copy operations will cause the inadvertent erasure of digital information embedded in halftone images.
Consequently, none of the above references provide a system and method for embedding invisible information into a document and reliably decoding the information at a later time.
Thus, there is a need to overcome these and other problems of the prior art and to provide an efficient method for embedding invisible information into an image and reliably decoding the information at a later time. The present invention, as illustrated in the following description, is directed to solving one or more of the problems set forth above.
In accordance with the present invention, a method is disclosed for encoding digital data in a hardcopy rendering of an invisible image defined by at least one circularly asymmetric dot pattern. The method comprises the steps of modulating the dot pattern in accordance with the digital data; and rendering the modulated dot pattern into a tiled halftone cell of predetermined color, size and pixel density on a recording medium, thereby producing a hardcopy rendering of the invisible image with the digital data encoded thereon.
In accordance with one embodiment of the present invention, the size of the cell is a 12xc3x9712 matrix.
In accordance with another embodiment of the present invention, the color of the dot pattern is yellow.
In yet another embodiment, the predetermined pixel density is 2%.